In the event of rehabilitation following any injury or simply in order to monitor and test an individual, it is important to ascertain the forces exerted by each of the legs of the individual when, for example, walking or running normally.
Apparatus is known which can be used to measure angular variations between the tibia and femur corresponding, in particular, to movements of flexion and extension when walking. There are a variety of methods and devices that have been described in the prior art for determining quantities related to the position, magnitude and distribution of vertical forces exerted by a subject's foot (or two feet combined) against a support surface during standing or walking. The three commonly used methods and devices include coupled force transducers, instrumented shoes, and independent force transducers.
A. Coupled Force Transducers
One class of methods and devices for determining quantities related to the forces exerted on a support surface uses a forceplate that typically is a flat, rigid surface that mechanically couples three but more often four linear force transducers. The typical forceplate includes linear force transducers coupled to a substantially rigid plate to form a single force measuring surface, and each provides a way by which the force measuring surface is used to quantify aspects of the forces exerted by the feet of a subject standing on the forceplate. The most commonly determined quantities used to describe the forces exerted on a standalone forceplate surface (i.e., not part of a treadmill) by an external body are the following: (1) the position (in the horizontal plane) of the center of the vertical axis component of force, (2) the magnitude of the vertical axis component of the center of force, and (3) the magnitude of the two horizontal axis components (anteroposterior and lateral) of the center of force. Calculation of position and magnitude quantities for the vertical axis component of the center of force requires that only the vertical force component be measured by each of the three (or four) mechanically coupled force transducers. To measure the horizontal axis components of force, the force transducers must also measure the horizontal plane components of force.
The exact form of the calculations required to determine the above described center of force position and magnitude quantities from the measurement signals of the linear force transducers depends on the number and positions of the force transducers. Specifically, these algorithms must take into account the known distances between the force measuring transducers.
When a forceplate is used to measure quantities related to the position of the center of force, the position quantity is always determined in relation to coordinates of the forceplate surface. If the position of the foot exerting the force on the surface is not precisely known in relation to the forceplate surface, or if the position of the foot changes with time relative to the surface, the position of the center of vertical force cannot be determined in relation to a specified anatomical feature of the foot.
In order to measure forces exerted by the foot, there are known systems which use a platform which rests on the floor and uses sensors. The platform is located along the path that is walked in order to obtain an image of the force exerted by a footstep. Nevertheless, it appears that such a solution is not satisfactory given the fact that the person has a natural tendency to pause (or at a minimum become self-conscious of the need to hit the forceplate and alter their gait) before walking onto the platform so that the force which is exerted is not natural. This system can be duplicated for each leg. This system is not suitable for the measurement of several consecutive steps, because different individuals have their own unique gait.
B. Instrumented Shoe
A second class of methods and devices described in the prior art for measuring quantities related to forces exerted by a foot against a supporting surface during standing and walking is a shoe in which the sole is instrumented with linear force transducers. The principles for determining the position of the center of vertical force exerted on the sole of the shoe by the subject's foot are mathematically similar to those used to calculate the position of the center of force quantities using a forceplate.
Because the position of an instrumented shoe is fixed in relation to the foot, the instrumented shoe can be used to determine the position of the center of vertical force in relation to coordinates of the foot, regardless of the position of the foot on the support surface. A disadvantage of the instrumented shoe is that the position of the center of vertical force cannot be determined in relation to the fixed support surface whenever the position of the foot on the support surface changes during the measurement process. Another disadvantage in a clinical environment is that the subject must be fitted with an instrumented shoe. Another disadvantage is that thin film transducers have been difficult to calibrate and are prone to folding and bending which result in spurious output. Also, only force normal to the film surface is measured, and forces in other directions go unmeasured. Also, because the inside of the shoe is unlikely to be flat, the precise direction of the measured force is indeterminate.
The position and the magnitude of the center of force exerted by a foot against the support surface are determined relative to anatomical features of the foot by embedding force transducers in the shoes of walking and running subjects. Measures of the timing of heel-strikes and toe-offs have been made using contact switches embedded in the subject's shoes.
C. Independent Force Transducers
A fundamentally different method and device described in the prior art for determining quantities related to the forces exerted on a standalone support surface utilizes a plurality of mechanically independent vertical force transducers. Each vertical force transducer measures the total vertical force exerted over a small sensing area. The independent transducers are arranged in a matrix to form a force sensing surface. The two-dimensional position in the horizontal plane and the magnitude of the vertical component of the center of force exerted on the sensing surface can be determined from the combined inputs of the mechanically independent transducers. When the vertical force transducers are not mechanically coupled, however, the accuracy of the center of vertical force position quantity will be lower, and depends on the sensitive area of each transducer and on the total number and arrangement of the transducers. When mechanically independent vertical force transducers are used to determine the position of the center of vertical force, the resulting quantities are determined in relation to coordinates of the force sensing surface.
The plurality of independent force measuring transducers can be used to determine additional quantities related to the distribution of forces exerted against a support surface by a subject's foot. Outlines of the foot can be produced by a system for mapping the distribution of pressures exerted by the foot on the surface. Usually the positions of anatomical features of the foot such as the heel, the ball, and the toes can be identified from the foot pressure maps. When the position of a first anatomical feature is determined in relation to the support surface by the pressure mapping means, the position of a second anatomical feature of the foot can be determined in relation to the support surface by the following procedure. The linear distance between the first and second anatomical features is determined. Then, the position of the second anatomical features in relation to the support surface is determined to be the position of the first anatomical feature in relation to the support surface plus the linear distance between the first and second anatomical features.
When a subject stands with a foot placed in a fixed position on the surface of a force sensing surface, the position of the center of force exerted by the foot can be determined in relation to coordinates of the forceplate surface. If the position of a specified anatomical feature of the foot (for example, the ankle joint) is also known in relation to the coordinates of the forceplate surface, the position of the center of force in relation to coordinates of the specified anatomical feature of the foot can be determined by a coordinate transformation in which the difference between the force and anatomical feature position quantities are calculated.
Forceplates, instrumented shoes and independent force transducers have all been used in the prior art to measure quantities related to the position and magnitude of the center of force exerted by each foot against the support surface during stepping-in-place, walking, and running. Forceplates embedded in walkways have measured quantities related to the position and magnitude in relation to the fixed (forceplate) support surface for single strides during over ground walking and running. Using additional information on the position of a specified anatomical feature of the foot in relation to the forceplate support surface, the position of the center of force has also been determined in relation to a specified anatomical feature of the foot.
Human gait may be classified in general categories of walking and running. During walking, at least one foot is always in contact with the support surface and there are measurable periods of time greater than zero during which both feet are in contact with the support surface. During running, there are measurable periods greater than zero during which time neither foot is in contact with the support surface and there are no times during which both feet are in contact with the support surface.
Walking can be separated into four phases, double support with left leg leading, left leg single support, double support with right leg leading, and right leg single support. Transitions between the four phases are marked by what are generally termed “heel-strike” and “toe-off” events. The point of first contact of a foot is termed a “heel-strike”, because in normal adult individuals the heel of the foot (the rearmost portion of the sole when shoes are worn) is usually the first to contact the surface. However, heel-strike may be achieved with other portions of the foot contacting the surface first. During running, normal adult individuals sometimes contact with the ball of the foot (forward portions of the sole when shoes are worn). Individuals with orthopedic and/or neuromuscular disorders may always contact the surface with other portions of the foot or other points along the perimeter of the sole when shoes are worn. Similarly, while the ball and toes of the foot are the last to contact the surface at a toe-off event in normal adults, a patient's last point of contact may be another portion of the foot. Thus, regardless of the actual points of contact, the terms heel-strike and toe-off refer to those points in time at which the foot first contacts the support surface and ceases to contact the support surface, respectively.
Treadmills allowing a subject to replicate walking and running speeds within a confined space have been described, for example, in U.S. Pat. No. 5,299,454 to Fuglewicz et al. and U.S. Pat. No. 6,010,465 to Nashner. A treadmill allows the difficulty of gait to be precisely set by independently controlling the belt speed and the inclination of the belt; however, prior art devices known to the inventors have not allowed for the slope to be changed from an incline to decline (or decline to incline) while an individual is using the treadmill. The subject can be maintained in a fixed position relative to the measuring surface underlying the treadmill belt by coordinating the speed of gait with the speed of the treadmill belt movement.
One method to determine the position of the treadmill belt on a continuous basis in relation to the fixed force sensing surface is to use one of several sophisticated commercial treadmill systems described in the prior art which measure the anteroposterior speed of the moving treadmill belt on a continuous basis, and which provide the means to regulate the belt anteroposterior speed on a continuous basis. When one of these treadmill systems is used, the information necessary to determine the continuous position of the treadmill belt in relation to the underlying forceplate is obtained by performing mathematical integration of the belt speed signal on a continuous basis.
There are methods described in the prior art which can be used to determine, at the time of heel-strike, the position of the moving treadmill belt in relation to the specified anatomical features of the foot. One method is to use one of several commercially available optical motion analysis systems. Two examples of commercially available motion analysis systems which describe applications for tracking the motions of identified points on the human body during locomotion include the ExpertVision system manufactured by MotionAnalysis Corp., Santa Rosa, Calif. and the Vicon system manufactured by Oxford Medilog Systems, Limited, Oxfordshire, England. In accordance with this method, one or more optical markers are placed on the specified anatomical features of the foot. One or more additional markers are placed on the treadmill belt at predetermined positions. The number and placement of the optical markers on the anatomical feature and the treadmill belt determine the accuracy of the measurement as specified by the systems manufacturers. At the time of heel-strike, the positions of the treadmill belt marker or markers are then determined in relation to the positions of the anatomical feature marker or markers in accordance with methods specified by the system manufacturer.
There have been numerous proposals and/or attempts to equip endless belts in an attempt to measure the loads applied when an individual walks. These systems involve fitting force meters between the base over which one side of the endless belt travels and the chassis. However, such proposals and/or attempts have several drawbacks. First, the measurement cannot differentiate between the force exerted by each leg; this poses relatively few problems when analyzing running motion because both feet practically never touch the ground simultaneously since contact is essentially one-footed, but it is an important shortcoming when the individual is walking because both feet always touch the ground since contact is two-footed as discussed above. Second, it is impossible to measure tangential forces in the x-axis and y-axis. Third, most studies have made a conscious decision not to try to capture the forces and torques in the horizontal plane caused by a footfall, probably given the relatively small contribution these forces have on the overall force analysis when compared to the vertical force.
U.S. Pat. No. 5,299,454 to Fuglewicz et al. and U.S. Pat. No. 6,010,465 to Nashner disclose a solution whereby the endless belt has a path around at least two forceplates in tandem. This solution has the inherent problem in that when the individual has both feet on the belt at the same time, the horizontal forces from one foot cancel out the horizontal forces of the other foot because the belt is pushed in opposite directions by the two feet. The other solution using a treadmill structure with multiple forceplates is discussed, for example, in U.S. Pat. No. 6,173,608 to Belli et al., which discloses a treadmill structure that has a pair of belts running in the longitudinal direction. The inherent problem with this structure is that the normal walking or running gait for people eventually places the feet one in front of each other such that the individual would have heel-strikes over the gap between the belts and thus register forces on both belts at the same time, which defeats the purpose of the device.
In light of the above drawbacks of the prior art described above, what is needed is a method and device for separately determining quantities related to the force exerted by each foot against the treadmill support surface at all phases of the step cycle.
Moreover, what is needed is a method for determining the forces and torques exerted on each foot as it moves from one surface of a treadmill to another. Such a method should calculate the location of these forces and torques on the treadmill surface.